The main SQLite developers have a company that has a few revenue streams. Mainly they offers support and maintenance contracts. There are also a couple of closed source extensions for SQLite for handling compressed and encrypted databases that they sell. Finally the complete test suite is closed source, so if you want to make closed source changes to SQLite and still make sure you don't break compatibility then you probably want to license that.
not going to say it is 100% of their time, but they are paid to work on it.
You can see five consortium members on the front page of sqlite, the cost of a consortium membership appears to $75,000/year[1]. That's enough to pay developers.
And that doesn't even include Apple, who I have to believe has a big investment in sqlite, and probably has some form of support arrangement with them.
